Residents of Gilbert, Ariz., check in to vote last November. / Mark Henle, The Arizona Republic

by The Editorial Board, USATODAY

by The Editorial Board, USATODAY

In recent years, too many states seem to have been more interested in imposing voter ID requirements than in making sure eligible voters weren't disenfranchised by them. The same may apply to citizenship tests.

After Arizona voted in 2004 to require voters prove their citizenship, more than 30,000 people were turned away because they didn't have proper documents, and voter registration in the county that includes Phoenix plummeted by 44%.

Some of those people might have been ineligible to vote, but all of them? Backers of strict voter requirements like to say that even one fraudulent vote undermines the legitimacy of the system. Fair enough. But the reverse is also true: Even one eligible voter unfairly kept from voting denies that person the most important right of any American. Both imperatives are crucial.

On Monday, the Supreme Court honored the second imperative when it blocked Arizona from requiring would-be voters to produce documents to prove their U.S. citizenship before they can be eligible to vote. But the victory could be short-lived.

The court, in its 7-2 ruling, said the state must yield to the less strict federal standard, which requires voters to swear on a "motor voter" form that they're citizens, under penalty of perjury. But the justices virtually invited Arizona and other states to come back to the courts and ask again.

Arizona is one of five states (Alabama, Georgia, Kansas and Tennessee are the others) that require voters to prove citizenship when they register. There's nothing wrong with states wanting to be sure voters are citizens, and it's entirely reasonable to worry that a determined non-citizen might not be scared off by the remote threat of a perjury indictment.

Our concern with this is the same as it has been when states impose tough photo ID requirements for voters, which about 30 states now do: The requirement should not be a backdoor way to suppress voting by the sort of people who typically don't have documents that most of us take for granted, such as a driver's license.